Magnetoresistive random access memory (MRAM) uses magnetic tunnel junctions (MTJs) to store data. An MTJ includes a pinned layer and a free layer separated by a dielectric layer. A relative orientation of the pinned layer and the free layer determines a resistance of the MTJ. The MTJ has a low resistance if the pinned layer and the free layer are oriented in a same direction. The MTJ has a high resistance if the pinned layer and the free layer are oriented in opposite directions. The pinned layer is fixed, so the relative orientation is determined by the free layer. The direction of the free layer is set by passing a high current through the MTJ. The direction of the current determines the orientation of the free layer.
A first MRAM bit cell, in an existing approach, includes an MTJ connected to a bit line and a pass gate transistor. The pass gate transistor selectively connects the MTJ to a source line. In order to perform a read operation on the first MRAM bit cell, a reference voltage is generated for comparison. Generating the reference voltage involves introducing additional circuitry and complexity to a memory array design.
A second MRAM bit cell, in an existing approach, includes two MTJs connected to a bit line and a bit line bar, respectively, and two pass gate transistors. Each of the two transistors selectively connects a corresponding MTJ to a source line. Both pass gate transistors are sufficiently robust to handle the entire current necessary to flip the free layer of each MTJ. The size of the pass gate transistors is a factor in determining the overall size of the MRAM bit cell.